It is common in the electrical wiring industry--such as domestic or industrial wiring and the like--to connect a plurality of wires in electrically conductive relation by using a twist on wire connector. Typically, twist on wire connectors comprise a plastic insulating cap and a coil of wire contained therein. The cap acts as a insulating housing around the coil and also provides a means for gripping the connector in order to twist it onto the wires. The coil comes into contact with the plurality of wires being connected. The gripping forces necessary to retain the wires in electrically conductive relation inside the coil are provided by both the coil and the cap. If the forces are provided by the cap, they are transmitted to the wires through the coil.
Some caps for twist on connectors used in the electrical industry today, may be made of thermosetting resin. Such types of material have a low modulus of elasticity and thus are not easily elastically deformable, together with a high resistance to plastic deformation--desirable characteristics of wire connectors. However, more recently it has become desirable to use thermoplastics, such as nylon, as the material used in the manufacture of connector caps for a variety of reasons, such as cost consideration in the production process and colour choice. Thermoplastics, however, are relatively elastic and therefore are easily deformed when tightened onto a pair of wires, generally tending to bulge around the periphery. It is possible to make such caps using an injection moulding process, but a cap configuration not specifically designed to be injection molded may provide problems for such manufacture, primarily due to cooling considerations.
Many presently available thermoplastic caps are thick walled with either small raised lines for gripping purposes, or thick walled with a plurality of thick ribs for gripping and twisting purposes. Most thin walled thermoplastic caps that are available employ an internal expansion coil which applies most of the retaining force used to bind the plurality of wires together. Such coils only contact the plastic connector cap at the ends of the coil and therefore transmit only a small fraction of the bursting forces to the cap. As a result, the cap does not need to be of high strength since the expansion coil bears the stresses involved. Thus the expansion coil needs to be of high quality, and therefore higher cost, than if it did not need to absorb the bursting forces.